Peter Sanchez, a regular visitor who took BART out from San Francisco in the hopes of taking a swim, looks on in dismay at the blue green algae that has forced the closure of Lake Temescal in Oakland, CA, Friday, July 25, 2014. less

Peter Sanchez, a regular visitor who took BART out from San Francisco in the hopes of taking a swim, looks on in dismay at the blue green algae that has forced the closure of Lake Temescal in Oakland, CA, ... more

Signs and caution tape warn visitors to Lake Temescal that a bloom of blue green algae has forced the East Bay Parks Department to close the popular summer attraction, in Oakland, CA, Friday, July 25, 2014.

Signs and caution tape warn visitors to Lake Temescal that a bloom of blue green algae has forced the East Bay Parks Department to close the popular summer attraction, in Oakland, CA, Friday, July 25, 2014.

Oakland's Lake Temescal will be closed to swimmers, waders and cannonball divers for the foreseeable future after a scuzzy layer of toxic blue-green algae has enveloped the lake, park officials said.

While blooms of algae are common at the popular North Oakland swimming hole, the decomposing algae in this particular bloom are producing a toxin called microcystin that can be harmful to humans, said Hal MacLean, water management supervisor for the East Bay Regional Park District.

"This one is different," MacLean said. "This one actually has a measurable level of toxin, so we had to close the lake. But is kind of weird, because they don't always produce it, so you can't know when it is going to happen."

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